Nerve cells, including pain receptors, extend axons from the central system to whereever they exist in the eventual adult. The tracks respond to the environment, which helps "steer" them to their destination. They get there the same way that other axons, such as the ones that run muscles, get where they need to be.

As other have said, pain is a way of getting the individual to stop doing something that is damaging them. We don't have pain receptors everywhere - some internal organs have virtually none. They are typically chemical detectors, responding to materials released from damaged cells and during inflammation, among other things, and chemical detection is an ancient ability.

"Tissues" means a couple of not-completely-compatible things - technically, it's a way to categorize cell types (there are only four or five basic animal tissue types), but not technically, it's the "stuff" we're made up of (which all contain multiple technical-tissue types).

a little important extra info: though the per se pain receptors are chemical, impulses from ALL receptors can be processed as pain by the brain if they are too intense. If you are asleep and the light is turned off and someone turns it on, your eyes hurt when you try to open them, don't they? There's visual pain for you.

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

MrMistery wrote:a little important extra info: though the per se pain receptors are chemical, impulses from ALL receptors can be processed as pain by the brain if they are too intense. If you are asleep and the light is turned off and someone turns it on, your eyes hurt when you try to open them, don't they? There's visual pain for you.

I would try to explain this as upsurge of electrical activity under some set of physical conditions which prevail when we are unconscious. This upsurge causes pain. And it goes away when we regain consciousness signifying change in the physical environment around the concerned neurons. Does it make sense ?

MrMistery wrote:a little important extra info: though the per se pain receptors are chemical, impulses from ALL receptors can be processed as pain by the brain if they are too intense. If you are asleep and the light is turned off and someone turns it on, your eyes hurt when you try to open them, don't they? There's visual pain for you.

the tissues in your eyes gets hurt by the intensity of light as the iris' opening is wide then the receptors tell the brain that its gonna do some damaging

actually the cause has to do with the adaptation of the eye to light. the darkness causes all the retinen in cone cells to associate with its opsin partner, leading to an increased sensitivity of the cell to intense light. When the light is turned on, it overwhelms the receptor. It takes a few minutes for the eye to adapt and to switch from rod cells to cone cells. I believe the term for this is receptor up-regulation.

But anyway, it was only an example to ilustrate my point.

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

not quite... The body has a function so that potentially harmful stimuli are interpreted as pain - that is the case of light. Also, when you touch a hot stove you don't burn your finger immediately, because the very high temperature alerts the body "Hey you idiot take your hand away from there, it's hot and will burn you"(not the best example cause that is a spinal reflex, but still you see my point)

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter